1. Field of the Invention
This invention relates to radio communication systems and methods, and more particularly relates to interference cancellation systems and methods for minimizing or eliminating interference in radio receivers due to unwanted signals. Even more specifically, this invention relates to a cancellation system and method which permits alignment of quadrature or phase position independently from the time match.
2. Description of the Prior Art
FIG. 1 is a functional diagram of a traditional interference cancellation system 1 having a reference input port 2 for providing a reference sample of an interfering signal, and a receiver antenna 4 for providing a receive signal on receiver transmission line 6 which couples the receiver antenna to a receiver 7. In order to obtain a sample of the interfering signal, reference input port 2 may be coupled to an auxiliary antenna (not shown), for example, if the receiver and source of the interfering signal are not collocated. Alternatively, the reference input port 2 may be directly coupled to the interfering signal source if the source and receiver are collocated. The received signal includes a desired signal component plus an interfering signal component.
The traditional interference cancellation system further includes a directional coupler 8 operatively connected to the reference input port 2 in order to provide an interfering signal sample on line 10 to a synchronous detector 12. The interfering signal sample is also provided via line 13 to a signal controller 14 which is operatively connected to the reference input port 2. The traditional interference cancellation system also includes a directional coupler 16 which is operatively connected to the receiver transmission line 6, for providing a sample of the received signal to the synchronous detector 12 via line 18.
The synchronous detector 12 receives both the interfering signal sample or reference signal via line 10 and a sample of the received signal via line 18. Thereafter, the synchronous detector compares the interfering signal sample with the received signal sample and essentially detects the portion of the received signal sample (i.e., the interfering signal component) that is coherent with the interfering signal sample. The synchronous detector 12 then provides DC output signals on its output ports which correspond to differences in amplitude and phase between the interfering signal sample and the coherent signal component of the received signal.
The traditional interference cancellation systems also include amplifiers and/or integrators 20 which are connected to the outputs of the synchronous detector 12 so that the DC output signals will be amplified and/or integrated to effectively create DC control signals, which are provided via lines 21, 22 to the signal controller 14.
The signal controller 14, also commonly known as a vector modulator, is regulated by the DC control signals provided by the synchronous detector 12 and integrator/amplifiers 20 of closed loops. As previously described, a first input of the signal controller is provided with an interfering signal sample from the reference input port 2 via line 13. Additionally, two other inputs of the signal controller receive the control signals from the integrator/amplifier 20 via lines 21, 22. The output port of the signal controller is operatively coupled to a directional coupler 23. A cancellation signal generated by the signal controller is provided on the signal controller output port to the directional coupler 23. Receiver transmission line 6 is operatively coupled to directional coupler 23 such that the cancellation signal is injected into the receiver transmission line carrying the received signal. Specifically, the cancellation signal, when injected into receiver transmission line 6, effectively cancels the interfering signal component from the received signal.
In the conventional interference cancellation system of FIG. 1, the signal controller (or vector modulator) and synchronous detector are typically quadrature devices. Quadrature vector modulators are commonly used in interference cancellation systems to adjust the amplitude and phase of a reference signal (such as on reference port 2) which is then injected as a cancellation signal into the receiver system to eliminate or minimize the effect of an interfering signal component in a received signal.
As previously noted, in order for the cancellation to take place, the cancellation signal from the signal controller and sample of the received signal must arrive at directional coupler 23 with the same amplitude and opposite phase (i.e., 180 degrees out of phase). This is commonly achieved by the synchronous detector 12. However, if this phase relationship is to be attained over a broad bandwidth, the time delay between the reference signal path and the sample of the received signal via directional coupler 16 to the synchronous detector 12 must be matched by the time delay between the two signals. A time mismatch of AT will cause a phase difference, .DELTA..phi., given by the following equation: EQU .DELTA..phi.=2.times..pi..times.F.sub.o .times..DELTA.T
where F.sub.o is the frequency of the interference signal and .pi. is approximately equal to 3.14. As noted above, the overall phase difference .DELTA..phi. can be made to approach zero.
However, at another frequency, F, there will still be a phase difference, as shown in the following equation: EQU .DELTA..phi.=2.times..pi.(F-F.sub.o).times..DELTA.T
Even though good cancellation may be achieved at frequency F.sub.o, the cancellation may be only partial at frequency F, or there may be no cancellation or even an enhancement of the interference signal received by the radio receiver.
This is illustrated by FIG. 2, which is a graph of the degree of cancellation as a function of frequency for a time mismatch of 1.0 nsec, as well as for 0.1 nsec. In the example shown in FIG. 2, the phase has been adjusted so that at frequency F.sub.o, the degree of cancellation is reduced. For example, at about 160 MHz or more from frequency F.sub.o, the interference signal is actually enhanced when the time mismatch is 1.0 nsec or greater. Accordingly, it is essential that there be a time match apart from amplitude match in achieving a high degree of cancellation across a defined bandwidth in an interference cancelling system. Such amplitude and time match is achieved by using an attenuating means such as a pad, and a delay line in the reference signal.
Thus, in a conventional interference cancellation system, a disadvantage of the system is that in most cases, the reference signal antenna and the receiver antenna must be spaced apart from one another so that there is a phase difference between the reference signal and the sample of the received signal taken from the receiver antenna. This phase difference is necessary so that the adaptive control loop of the cancellation system and in particular the synchronous detector 12 of the loop, can distinguish between the two signals and provide a proper detector output signal to the signal controller.
Another disadvantage of conventional cancellation systems is that they depend upon very accurate matching the propagation delay time of the reference signal path and the sample of received signal. In addition, the relative phase of the reference signal path and the cancellation or error signal path must be adjusted for stable loop performance by aligning the internal phase quadrature of the system (i.e., a positive incremental change in the output of the Q integrator 20 must cause a positive change in the output of the synchronous detector 12 for an inverting integrator design). The internal phase quadrature of the system will hereto be referred as quadrature position. For stable, optimum performance, the quadrature position variation with frequency should be minimized. Most times, due to dissimilar components in the reference signal path and error signal path, these two requirements are conflicting. The final result is a compromise on time match in order to meet the primary requirement of loop stability.